Two members of a legendary Russian ballet family arrived here last night after defecting in Japan and thanked Soviet Embassy officials in Tokyo for their "understanding."

Dancer Mikhail Messerer, 31, told reporters at Kennedy Airport that Soviet Embassy officials in Tokyo did not try to stop him when he defected with his 71-year-old mother Tuesday because she is ill.

"My mother is not feeling well and Japanese specialists told her that she would be well taken care of by high-quality American specialists," Messerer said through an interpreter. "Of course, I can't leave her alone."

The slender, dark-haired dancer, who did not disclose the nature of his mother's illness, then declared unexpectedly:

"I would like to thank the workers in the Soviet Embassy (in Tokyo) for their understanding of our problem.

"They didn't do anything to stop us from coming from Japan to the United States."

His mother, Sulamif Messerer, confirmed this in a brief statement she made in heavily accented English.

"The Soviet ambassador didn't do anything against us," she said.

The dark-haired petite Mrs. Messerer, a former Bolshoi star who is now a teacher, said she was happy to be in the United States but "a little bit tired" after the 13 1/2-hour flight from Toyko.

Messerer said he and his mother "hope that we can resume our work" in the United States.

Earlier, in Tokyo, a Japanese Foreign Ministry spokesman said the two had defected because they wanted greater artistic freedom. They were granted asylum after spending Tuesday in protective custody of the U.S. Embassy, where they sought assistance in defecting.

Messerer, a widow and frequent visitor over the past 20 years who is fluent in Japanese, had been in Tokyo since last November as head of the Bolshoi Troupe giving ballet lessons to members of the Tchaikovsky Memorial Tokyo Ballet Troupe under contract. Sources said she has the honorary title of Soviet People's Artist and has many ballet students in Japan.

Moscow ballet fans considered Mikhail Messerer -- who arrived in Japan with the Bolshoi on Jan. 24 -- a capable dancer but not one of the troupe's stars. He had appeared recently in small solo roles in Moscow in the 2-year-old ballet "These Enchanted Sounds." This was his fourth visit to Japan.

The Kyodo news service said Messerer apparently had planned her defection because she had been depositing her savings in a foreign bank in Tokyo. The newspaper Asahi Shimbun quoted police sources as saying the two "despaired life in Russian society."

John Ohta, U.S. Embassy spokesman, said the Messerers "wanted a freer environment for artist activities." The Soviet Embassy declined to comment, saying no officials were available.

The nearly 100-member Bolshoi troupe left Moscow on Jan. 23 for a month-long tour of 12 cities in Japan. Its first performance was in Tokyo last Saturday. It gave its fifth performance Wednesday before a capacity crowd in Nagoya, in central Japan.

Sources close to the Bolshoi said troupe members were given security screenings before being allowed to leave Moscow for Japan. The sources said 11 or 12 troupe members were barred from making the trip, including star male performer Vladimir Vasiliev.

The screenings appeared to be a result of sensitivity of Soviet authorities over the possibility of further defections after the three last summer in the United States.

The Messerer defections may account for the sudden announcement in Paris last night that a scheduled Bolshoi visit to France was postponed without explanation.

The Bolshoi has been plagued by a string of defections in recent months, including that of the company's principal male star Alexander Godunov in New York last August. Leonid Kozlov and his dancer wife Valentina slipped away from the company in Los Angeles two weeks later.

A run of recent defections also includes world and Olympic skating champions Oleg Protopopov and Ludmila Belousova, who asked for political asylum in Switzerland last September.

Few families were as closely or as extensively involved with the Bolshoi and the world of Soviet culture in general as the Messerers, a Jewish family which moved from Latvia to Russia in the 19th century.

Mrs. Messerer's brother, Asaf, is still regarded at the age of 76 as the Bolshoi's principal ballet master. He and she danced together in many leading roles at the Bolshoi Theater in the 1930s and became favorites of Stalin.

Mrs. Messerer is the aunt of the Bolshoi's best known prima ballerina, Maya Plisetskaya, whose brother Azary has been training the Cuban National Ballet.

Mikhail has danced several solo roles with the Bolshoi although he is not considered one of the troupe's top stars. Mikhail's brother Boris, an artist, is married to poet Balla Akhmadulina, who last week publicly defended exiled dissident leader Andrei Sakharov.

The family has also provided many well-known Soviet doctors, lawyers and painters.

A spokesman for the Tokyo Ballet Company said Mrs. Messerer was considered the founder of postwar classical dance in Japan and "is really famous among the Japanese ballet circle."

A Soviet weekly recently published an article on her work with the Japanese ballet, praising her for being an emissary of Soviet culture in Tokyo.